The total capacity of the Li-ion element is constituted by the capacities of the anode and cathode. The specific capacity of the cathode is generally approximately 50% lower than the specific capacity of graphite carbon based anode. As finding suitable cathode materials with larger capacity is rather difficult and the list of such materials is limited, the main condition for developing Li-ion elements with high-efficiency is to employ anode material with a remarkably higher capacity than that of graphite. While the theoretical special capacity of graphite is 372 mAh g−1, then a considerable improvement of the performance of the Li-ion element would be provided by an anode, the special capacity of which is at least 1000 mAh g−1. Possible chemical elements forming alloys with lithium as potential anode materials have been searched from elements such as Sn, Pb, Al, Au, Pt, Zn, Cd, Ag, and Mg. However, the highest potential is seen in silicon (Si), which is on the second place based on its prevalence on earth. In Si—Li alloys, silicon can contain up to a maximum of 4.4 Li atoms that corresponds to the chemical composition Li22Si5 and where the theoretical insertion capacity of lithium is 4200 mAh g−1 [1].
Another advantage of silicon anode, in addition to its high theoretical special capacity, is the fact that it works in a narrow range of potentialities between 0.0 V and 0.4 V, which is suitable for working in the Li-ion element. The coulomb efficiency of the recharging-emptying cycle of pure Si anode is only ˜35%, and the electrode stops functioning already during a few cycles. The reason lies in the fact that major volume related changes take place in the electrode during the recharging-emptying process. The volume of the Li22Si5 alloy per one Si atom is 4 times greater than that of the initial Si atom and corresponds to a 400% expansion of the volume of Si grid. This causes the electrode to crack and resolve which results in the loss of the so-called active mass—decrease in electronic contact and fading of capacity.
To overcome the problems resulting from major volume-related changes, increase coulomb efficiency and improve the cycleability, i.e. the lifespan of Si anodes, several methods have been used [2]. One option is to use Si micro- and nanopowders in anodes [3], in which the spaces between particles buffer the expansion of Si particles. Another option is to incorporate Si particles in active or inactive matrix into composite electrodes [4]. The most common matrix material is carbon, which buffers the volume-related effects in electrodes resulting from the chemical processes on Si surface.
Si/C composite electrode material can be made, for example, from the pyrolysis of various polymethyphenylsiloxanes at 900-1300° C. [4]. Such electrodes have a reversible capacity of ˜550 mAh and an irreversible capacity of ˜300 mAh whereas the general tendency is that while the temperature during pyrolysis is rising, the reversible capacity increases and irreversible capacity decreases. Such an effect is associated with complete removal of oxygen and formation of SiC at a higher temperature. Also, siloxane polymers made from the alloy of various multiprecursors have been tested as anode materials; it has appeared that with these the rise of oxygen level in composite electrodes increases irreversible capacity and discharging potential [5].
Epoxy silane composites, the pyrolysis of which at 1000° C. creates a carbon of unstratified graphite sheets with a disordered structure, have been studied for determining a suitable Si/C relationship [6]. In certain cases, such composites have a reversible capacity of ˜770 mAh g−1, but a crucial disadvantage is a large proportion of irreversible capacity. Analogous results have been obtained also with resin-polysilane composite anodes.
As the presence of oxygen in the composite increases the irreversible capacity, oxygen-free compounds have been tested as starting materials for both silicon as well as carbon components—for example benzene, SiCl4 and (Me)2Cl2Si. The characteristic capacity features of such material are reversible capacity of ˜640 mAh g−1 and irreversible capacity of ˜120 mAh g−1. The Si/C composite electrode can be formed by pyrolytic disintegration of silane in carbon microbeds [7]. However, such electrode has a very low coulombic efficiency (45%) compared to a corresponding pure carbon material (77.5%), which is believed to result from the formation of SEI (solid electrolyte interface) layer, the binding of irreversible Li+ and the decrease of active material during cycling.
However, if nano-Si is precipitated during pyrolytic processing into Timcal K-6 graphite, it results in an anode with excellent cycling properties [8]. After a 100th cycle the capacity is reduced by ˜1%.
Plasma precipitation method was used to prepare a thin silicon membrane coated with fullerene (C60) [9], which, after 50 cycles, had a special capacity of 2000 mAh g−1 (with current density 500 μA cm−2). An excellent cycleability of such material is found on a polymeric fullerene layer that protects against SEI formation.
In U.S. Pat. No. 5,834,138, Yamada et al, Nov. 10, 1998, a material of negative electrode with solid electrolyte that contains material with sintered and carbonized molecular weight, additionally containing a metallic element, has been described. Whereby the said material is able to chemically bind and release lithium ions.
In U.S. Pat. No. 5,451,477, Sony Corp, Sep. 19, 1995, a storage battery with solid electrolyte, which contains anodes made from carbon material that is able to bind and release lithium ions, has been described. The carbon material is made of graphite, consisting also non-graphitic carbon.